Introduction to Computing Principles - - PowerPoint PPT Presentation

天津大学 计算机科学与技术学院 刘志磊

Introduction to Computing Principles

计算原理导论

1

Software: code that runs on the hardware

2

What does “run” means?

3

How does that actually work?

Introduction to computing principles computer software

• CPU implements "machine code" instructions
• Each machine code instruction is extremely simple
• e.g. add 2 numbers
• e.g. compare 2 numbers
• Javascript code we've used: pixel.setRed(10)
• Javascript is not machine code, so it does not run on the CPU directly.
• It must be translated to ~10 machine instructions to actually run on the

CPU (detail on this later)

Software Machine Code Instructions and Programs

Introduction to computing principles computer software

• A CPU understands a low level "machine code" language
• also known as "native code"
• Machine code is hardwired into the design of the CPU hardware
• it is NOT something that can be changed at will.
• Each family of compatible CPUs (e.g. the very popular Intel x86

family) has its own, idiosyncratic machine code which is not compatible with the machine code of other CPU families.

Software Machine Code Instructions and Programs

Introduction to computing principles computer software

• The machine code defines a set of individual instructions.
• Each machine code instruction is extremely primitive (原始的)
• such as adding two numbers or testing if a number is equal to zero
• When stored, each instruction takes up just a few bytes.
• A CPU can execute 2 billion operations per second
• "operations" refers to simple machine code instructions.

Software Machine Code Instructions and Programs

Introduction to computing principles computer software

• “Program” (程序)
• e.g. Firefox/Chrome, millions of simple machine code instructions
• Instructions, like grains of sand making up sculpture
• CPU runs a "fetch/execute cycle"
• fetch one instruction in sequence
• execute (run) that instruction, e.g. do the addition
• fetch the next instruction, and so on
• Some instructions may affect the order that the CPU takes

through the instruction sequence

What is a Program? What is Running?

Introduction to computing principles computer software

• "loop" instruction
• jump back 10 instructions
• Loops are implemented this way
• "if" instruction
• skip ahead if a certain condition is true
• If statements are implemented this way

What is a Program? What is Running?

Introduction to computing principles computer software

What is a Program?, What is Running?

• A program, like Firefox.exe (.exe is a Windows

convention), which is a big collection of bytes

• The Firefox.exe bytes are basically the millions of

instructions

• Double click Firefox.exe to Run
• The instruction bytes are copied up into RAM
• The CPU is directed to start running at the first instruction

Run a Program

Introduction to computing principles computer software

What is a Program?, What is Running?

Run a Program Who did this?

Introduction to computing principles computer software

Operating System Boot / Reboot Computer Languages

• Who starts Firefox?
• Operating System: Windows, Linux, Android, iOS
• Set of supervisory programs run when computer first starts
• Invisible administration behind the scenes
• Starting/managing/ending other programs
• Multitasking: run multiple programs at the same time
• Operating system keeps each program run isolated
• Program has its own RAM, its own windows on screen
• vs. accidental or malicious action between programs
• e.g. Laptop, Digital camera

Introduction to computing principles computer software

• Who runs the operating system?
• Chicken and egg problem..
• When first powered on, computer runs a tiny "get started" program.

That program typically looks for a disk containing an operating system to run

• Boot up (启动)
• Start
• Reboot （重启）
• shutdown/start-fresh cycle

Operating System Boot / Reboot Computer Languages

Introduction to computing principles computer software

• From Programmer to the CPU:
• We've written small Javascript programs
• We've seen large program like Firefox
• Computer language used by a person (e.g. Javascript)
• vs. the simple machine code instructions in the CPU
• What's the connection?
• Here the basic themes, not the details

Operating System Boot / Reboot Computer Languages

Introduction to computing principles computer software

• It is extremely rare to write machine code by hand.
• Instead, a programmer writes code in a more "high level"

computer language with features that are more useful and powerful than the simple operations found in machine code.

• For CS101, we write code in Javascript which supports high level

features such as strings, loops, and the print() function.

• None of those high level features are directly present in the low

level machine code; they are added by the Javascript language.

• There are two major ways that a computer language can work.

Operating System Boot / Reboot Computer Languages

Introduction to computing principles computer software

What is a Program?, What is Running?

Source Code and Compiler

• Computer languages
• "high level" features
• e.g. loops, if-statements, strings
• e.g. C, C++, Javascript, Java
• Programmer writes "source code" of a program in a language
• How to get program to the CPU?
• Having the .exe allows one to run the program
• To add feature or fix a bug, ideally you want the source code
• Add a feature in the source code, then run the compiler again to make a

new version of the .exe

Introduction to computing principles computer software

• Compiler (编译器) translates the source code into a large

number of machine code instructions

• Suppose a high level construct, like an if-statement, can be

implemented by a sequence of 5 machine code instructions

• e.g. Firefox: written in C++, compiler takes in Firefox C++ source

code, produces Firefox.exe

• The compilation step can be done once.
• The end user does not need to the source code or the compiler.
• Does not work backwards: having the .exe, you cannot recover the

source code (well)

Source Code Compiler

Introduction to computing principles computer software

• The source code (源代码) is available to all, along with the

right to make modifications

• Typically the software does not cost anything
• Freedom: the end user is not dependent on the original vendor to fix

bugs, or perhaps the vendor goes out of business

• The user can make the change themselves (since they have access to

the source)

• Insurance/freedom policy
• Often the license terms will require the change to made available to the

wider community in some cases

• Open source is a very successful model for some cases

Open Source Software

Introduction to computing principles computer software

• “Open Source“ (开源) refers to software where the

program includes access to its source code, and a license where the user can make their own modifications. Typically open source software is distributed for free.

• Open

source software also includes freedom

• r

independence since the user is not dependent on the

• riginal vendor to make changes or fixes or whatever to

the source code.

Open Source Software

Introduction to computing principles computer software

Dynamic (interpreter) Languages

• Dynamic languages
• e.g. Java, Javascript, Python
• Does not use the compiler/machine-code strategy
• Can be implemented by an “interpreter“ (解释器)
• Interpreter is a program which "runs" other code
• e.g. web browsers includes a Javascript interpreter, browser "runs" bits
• f Javascript code in a web page
• Interpreter looks at one line at a time
• Deconstructs what each line says to do
• The interpreter then does that action in the moment
• Then proceeds to the next line

Disclaimer: there are many languages, no one "best" language, it depends!

Introduction to computing principles computer software

Interpreters & Compilers

• Interpreter (解释器)
• A program that reads a source program and produces the results
• f executing that program
• Compiler (编译器)
• A program that translates a program from one language (the

source) to another (the target)

• The compiler translates source code to equivalent machine code
• The interpreter does the code, looking at each line and doing it

Introduction to computing principles computer software

• Compiled code tends to run faster than Interpreter code
• The compiler has pre-processed the source code, leaving the

program.exe as lean (精干) and direct as it can be to just run.

• many questions are resolved by the compiler at compile time
• how to append to this string, how many bytes do I need here…
• Dynamic/interpreter languages
• Tend to have a greater number of programmer-friendly features
• e.g. Memory Management （存储管理）
• C and C++: partially manual, some programmer input required
• Dynamic languages: automatic memory management, no programmer input needed
• Automatic memory management not free: spending CPU cycles to lighten programmer

workload

• Programmers are often more productive in dynamic languages
• The resulting program tends to run slower than compiled code

Interpreter Compiler vs. Interpreter Evaluation Tradeoff Current trend is towards dynamic languages  Moore’s Law

Introduction to computing principles computer software

Program Running

• Just In Time Compiler (JIT, 即时编译器)
• Compile code of a dynamic language on the fly
• The interpreter is used for simple cases
• For important sections of dynamic code, the JIT creates a block of

machine code in RAM for that section

• All major browsers now have a JIT for the Javascript code they run
• Best of both worlds
• Flexibility of dynamic languages
• Combined with most of the performance of the compiled world
• Active area of research, works pretty well

Dynamic Languages Just In Time Compiler Program Running

Introduction to computing principles computer software

Program Running

Source code Compiler Assembly code Assembler Object code (machine code) Fully-resolved object code (machine code) Linker Loader

Normally the program building process involves four stages and utilizes different ‘tools’ such as a preprocessor (预处理器), compiler (编译器), assembler (汇编器), and linker (链接器).

Dynamic Languages Just In Time Compiler Program Running

Introduction to computing principles computer software

The right Figure shows the steps involved in the process of building the C program starting from the compilation until the loading of the executable image into the memory for program running.

Why Memory Management?

Dynamic Languages Just In Time Compiler Program Running

Introduction to computing principles computer software

• Program must be brought (from disk)

into memory and placed within a process （进程） for it to be run

• If only a few processes can be kept in

main memory, then much of the time all processes will be waiting for I/O and the CPU will be idle (空闲)

• Hence, memory needs to be allocated

efficiently in order to pack as many processes into memory as possible

Running time of a user program Dynamic Languages Just In Time Compiler Program Running

Why Memory Management?

Introduction to computing principles computer software

How to do memory management？  Binding of Instructions and Data to Memory

• Address binding of instructions and data to memory addresses can

happen at three different stages

• Compile time: If memory location known a priori, absolute code can be

generated; must recompile code if starting location changes

• Load time:

Must generate relocatable code if memory location is not known at compile time

• Execution time:

Binding delayed until run time if the process can be moved during its execution from one memory segment to another. Need hardware support for address maps (e.g., base and limit registers)

Dynamic Languages Just In Time Compiler Program Running

Introduction to computing principles computer software

Logical vs. Physical Address Space

• The concept of a logical address space that is bound to a separate

physical address space is central to proper memory management

• Logical address (逻辑地址) – generated by the CPU; also

referred to as virtual address (虚地址)

• Physical address (虚地址) – address seen by the memory unit
• Logical and physical addresses are the same in compile-time and

load-time address-binding schemes;

• logical (virtual) and physical addresses differ in execution-time

address-binding scheme

Dynamic Languages Just In Time Compiler Program Running

Introduction to computing principles computer software

Memory-Management Unit (MMU)

• Hardware device that maps virtual to physical address
• In MMU scheme, the value in the relocation register (寄存器) is

added to every address generated by a user process (用户进程) at the time it is sent to memory

• The user program deals with logical addresses; it never sees the real

physical addresses

Dynamic Languages Just In Time Compiler Program Running

Introduction to computing principles computer software

Dynamic relocation using a relocation register

Dynamic Languages Just In Time Compiler Program Running

Introduction to computing principles computer software

Dynamic Loading

• Routine/Program is not loaded until it is called
• Better memory-space utilization
• Unused routine/Program is never loaded
• Useful when large amounts of code are needed to handle in

frequently occurring cases

• No special support from the operating system is required

implemented through program design

Dynamic Languages Just In Time Compiler Program Running

Introduction to computing principles computer software

Dynamic Linking

• Linking postponed until execution time
• Small piece of code, stub, used to locate the appropriate

memory-resident library routine

• Stub replaces itself with the address of the routine, and

executes the routine

• Operating system needed to check if routine is in processes’

memory address

• Dynamic linking is particularly useful for libraries
• System also known as shared libraries

Dynamic Languages Just In Time Compiler Program Running

Introduction to computing principles computer software

Swapping

• A process can be swapped temporarily out of memory to a

backing store, and then brought back into memory for continued execution

• Backing store – fast disk large enough to accommodate

copies of all memory images for all users; must provide direct access to these memory images

• Roll out, roll in – swapping variant used for priority-based

scheduling algorithms; lower-priority process is swapped

• ut so higher-priority process can be loaded and executed
• Major part of swap time is transfer time; total transfer time

is directly proportional to the amount of memory swapped

• Modified versions of swapping are found on many systems

(i.e., UNIX, Linux, and Windows)

• System maintains a ready queue of ready-to-run processes

which have memory images on disk

Dynamic Languages Just In Time Compiler Program Running

Introduction to computing principles computer software

Schematic View of Swapping

Dynamic Languages Just In Time Compiler Program Running

Thank You! Q&A